Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p21c..05k&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P21C-05
Mathematics
Logic
1040 Isotopic Composition/Chemistry, 6225 Mars
Scientific paper
A lander mission to an Archean terrane exposed on Earth today would be hard-pressed to find evidence of ancient microbial life. Exhaustive field sampling and petrographic examination of hundreds of thin sections usually yield only controversial examples of rare microfossils. C isotope evidence is similarly controversial and requires knowledge of the planetary C isotope reservoir and how it is partitioned, knowledge only partially achieved for Earth after 40 years of unconstrained sampling. Organic molecule biomarkers are fraught with questions of degradation and more recent contamination. If such problems exist for past life detection on Earth, why offer hope for success on limited missions to Mars? Circumstantial evidence suggests that microbial life became established on Earth within 1 Ga. It appears possible that Mars had widespread surficial water within this time frame, so evidence of past life there is not inconceivable. Unlike Earth, Mars froze down and has apparently not endured the kind of aggressive hydrologic cycle that has eradicated so much evidence of early life on Earth. Any martian sedimentary minerals have likely undergone minimal diagenesis/recrystallization and thus could potentially house preserved microfossils on a scale unknown on Earth. Tectonic deformation and metamorphism of surficial materials are restricted to local areas of more recent volcanism or late impacts. Widespread preservation of any water-laid early sediments is thus likely to be superb. Although the outlook for preservation is excellent, prospects for early life must confront a salinity issue regarding martian hydrospheric evolution. Any early hydrosphere on Mars would have started with dissolved NaCl significantly higher than that of terrestrial sea water. This would have subsequently evolved into a concentrated Ca,Mg,NaCl brine during evapoconcentration due to atmospheric water loss followed by megaregolith/ brine reactions and then eutectic freezing. The salinity issue is unfavorable unless salt tolerance is an attribute of earliest life. If not, the search for past microbial life on Mars requires the unlikely possibility that more dilute water masses were somehow generated and maintained for time intervals long enough to evolve life. In arid or semi-arid land regions of Earth, CaCO3 develops readily on basalts during weathering and can entomb organic molecules and microfossils. Correlated delta 18O - delta 13C variations in this "caliche" in surficial vs soil samples constitute a compelling C isotope biosignature, possibly the only one that does not require prior knowledge of the overall C isotope reservoir. On Mars, such material should be a prime exploration target because of the ease with which white, soft calcite can be recognized and sampled. Being a result mainly of precipitation, it would have formed in a long-lived (albeit intermittent) supply of less saline water in which bacteria less tolerant of salt could have evolved. Carbonates in ALH 84001 may be an example of shock metamorphosed caliche. If so, caliche development on Mars could be common. Caliche can be expected to elude detection by remote sensing and lander photos because it is easily removed from surfaces by wind abrasion. Internal vesicles, fractures, and the undersides of rocks protect caliche and remain excellent target areas for astrobiological prospecting.
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